Here is the code for populating a DropDown based on selection of another drop down list in Details view control of ASP .NET , in this example i've added two DropDowns in DetailsView control using TemplateField and InsertItemTemplate
The Second DropDown (ddlProducts) is getting populated based on Category selected in Category DropDown , For this i've used SelectedIndexChanged event and findControl method to find the control and sqldataSource is used as datasource for the dropdowns and DetailView